Devices such as firewalls are sometimes used to prevent users from accessing resources to which they are not authorized. As an example, members of the public may be entitled to access content served by a web server, but not authorized to access other services available on the server such as administrative tools. In another example, employees of a company may be entitled to access certain websites or certain class of websites while other websites or other classes of websites may be prohibited for all employees. Firewalls and other security devices typically enforce policies against network transmissions based on a set of rules.
To implement complex security policies, a firewall needs to keep track of many independent events and correlate the events for policy enforcement. Firewalls or other security devices typically maintain event statistics using counters which are updated rapidly to effectively examine network traffic as the traffic is being communicated.